Ultrasonic bonding or welding is a metallurgical joining technique which utilizes high frequency vibrations disrupting the surface films and oxides thereby promoting interatomic diffusion and flow between the surfaces in contact. Briefly stated, the ultrasonic bonding or welding process consists of clamping or otherwise securing together selected work pieces under moderate pressure between a bonding or welding tip and a support and then introducing high frequency vibratory energy into the tip for a relatively short period of time, i.e. from a fraction of a second to a number of seconds. A brief application of ultrasonic vibratory energy in a direction substantially parallel to the workpiece surfaces makes the bond. The lateral motion of the first workpiece surface over the second workpiece surface breaks up films thereon to provide a nascent bonding surface. The parameters of force, time, temperature and ultrasonic energy generate heat at the interface of the workpiece surfaces, combine to create a nascent surface, and produce a true metallurgical bond. The key to the success of this process is the use of unique bonding tools designed to transfer ultrasonic energy and heat into the workpieces efficiently and repeatedly.
The combined use of heat and ultrasonic vibrations for the purpose of sealing, bonding or welding two or more similar or dissimilar materials was initially limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important.
The use of ultrasonic bonding has been accompanied by certain problems. Those problems include, while not necessarily limited to, inefficient transfer of ultrasonic energy to selected workpieces and difficulty in establishing, controlling, and maintaining an optimal temperature of the bonding tip to ensure repeatable satisfactory resultant bonds. To that end, improvements in bonding tools to maximize ultrasonic energy transfer include those disclosed in U.S. Pat. No. 4,776,509 and U.S. Pat. No. 4,778,097. Thermally assisted ultrasonic devices employing, for example, an electric resistance heating element (U.S. Pat. No. 4,529,115) or a laser light source (U.S. Pat. No. 4,534,811) were developed.
Referring to U.S. Pat. No. 4,776,509, improvements to a bonding tip include a multiple of nonparallel surfaces extending from the end of the tip for maximizing the amount of ultrasonic energy coupled to the electrical conductors or workpieces. The surfaces may be recessed into the tip or protrude outwardly from the end of the tip for coupling ultrasonic energy in directions both parallel and perpendicular to the conductors. In U.S. Pat. No. 4,778,097, the bonding tip provides a channel or groove for holding the wire against the pad. The tip has a central concave surface whose cross-sectional shape approximates a minor portion of the wire's cross-sectional shape both as to geometry and as to dimension.
Referring to U.S. Pat. No. 4,529,115, as representative, conventional thermally assisted ultrasonic bonding devices typically include an electric resistance coil wrapped around the bonding or welding tip. The resistance coil serves to thermally assist in establishing and maintaining an optimum welding tip temperature. In order to insure and provide for maximum heat transfer between a resistance coil and welding tip, kapton tape is provided to cover and isolate the resistance coils. However, a limitation of this reference is that the thermal mass being heated is very large compared to the specific area of the desired bond or weld. The efficiency of the heat transfer between resistance coil and welding tip is limited, in part, to the integrity of the insulating tape. In addition, an elevated static temperature of the thermal mass poses a safety hazard, or can be potentially degrading to the operability of adjacent or integral heat sensitive components. The cumulative result of these limitations is that a thermally enhanced ultrasonic welding arrangement of this sort does not provide a desirable bonding for fine or superfine wires, for optical device components or for devices having heat sensitive circuit elements.
Referring now to U.S. Pat. No. 4,534,811, as representative, a conventional laser-assisted ultrasonic bonding device (sometimes referred to as laser-sonic bonding device) is implemented using a laser and a hollow ultrasonic bonding tip. The combination of the laser and the ultrasonic bonding tip offers the ability to dynamically provide localized heat in a short pulse at a limited bond area. The combination of ultrasonic bonding along with dynamic laser heat avoids several of the shortcomings of the thermally assisted implementation of U.S. Pat. No. 4,529,115. Namely, the thermal mass being heated may be significantly smaller inasmuch as an optimal temperature is attained via dynamic means through laser pulses. Tape, or other supplemental insulating or bonding means, is not required. In addition, the dynamic application of optimal heat for the purpose of welding or bonding minimizes the probability of damage to adjacent or integral heat sensitive devices. However, the expense and complexity of a laser is high, the tips are very expensive and incompatible with some bonding processes, and optical fibers used to transmit the laser source must be repolished approximately after every 1000 bonds.
There is a need in the art, therefore, for a bonding tip providing required bonding energy without sustained heating of the bonding tip thermal mass that may result in damage to adjacent heat-sensitive components. The needed bonding tip is preferably one that is usable with a known bonding method or apparatus. The needed bonding tip further is preferably relatively inexpensive to manufacture. Embodiments of the various aspects of the invention addressing these needs will now be described, by way of drawings and examples.